partitioner_test: stop calling set_global_partitioner
partitioner_test: stop calling set_global_partitioner

All the places that use partitioner have been switched
to not use global partitioner any more and we can stop
setting it in this test.

Signed-off-by: Piotr Jastrzebski <2c0eb4a25a7cde333ff2d0860cb06051925fd74f@scylladb.com>

avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla